Power converters are used in a variety of situations, to convert voltages from one value or type to another. They can operate to convert an AC voltage to a DC voltage (e.g., a rectifier); to convert a DC voltage to a different DC voltage (e.g., a flyback converter); to convert an AC voltage to a different AC voltage (e.g., a frequency changer); or to convert a DC voltage into an AC voltage (e.g., an inverter). In its simplest form, a power converter includes a transformer with a main primary winding for receiving a power signal to be converted, and a main secondary winding for providing a converted power signal. But many power converter devices also include additional windings that provide additional converted power signals. Such additional windings can be primary windings or secondary windings, as required by the parameters of the device design.
Switched mode power converters, in particular, are often used since they are typically smaller, more efficient, and produce less heat than linear power converters. A switched-mode power converter runs a DC input signal through a high frequency power oscillator (e.g., implemented using a transformer). In such a power converter, the oscillation switching is typically implemented using a MOSFET switch connected between the main primary winding on the converter's transformer and ground. When the MOSFET switch is closed in a flyback converter, the current through the main primary coil of the transformer rises, increasing the energy stored in the transformer; and when the MOSFET switch is opened, the current through the main primary coil of the transformer decreases as the energy stored in the transformer is transferred to the outputs of the power converter.
One common multiple-output, switched mode power converter includes a main primary winding for receiving a power signal to be converted, a main secondary winding for providing a converted power signal to a load, and an additional primary winding for providing a power signal for a primary side controller (e.g., a primary side bias output). This primary side controller is the circuit that controls the operation of the MOSFET switch connected to the main primary winding. Other types of multiple-output power converters can also have additional secondary windings that provide additional converted power signals to additional loads.
Often a switched power converter will operate in a continuous conduction mode (CCM). In this operation mode the current through the main primary winding will rise when the MOSFET switch is closed and fall when the MOSFET switch is opened, but will never fall to zero. This will often occur during normal or high load operation for the device.
In order to conserve energy during light load operation of a switched mode power converter, however, the converter is allowed to enter into a discontinuous conduction mode (DCM) of operation. The DCM operation is characterized by a switching interval that is the result of natural commutation in an output rectifier stage caused by a diode or diodes preventing the reversal of current in a transformer or inductor. The DCM operation occurs naturally at sufficiently light loads for converters that have only diodes in their output rectification stage. Converters with synchronous rectifiers can only reach DCM if the load is sufficiently light and the synchronous rectifier is disabled. In DCM operation, the average switching frequency of a control MOSFET is often reduced to maintain high efficiency by reducing switching losses. The DCM operation is characterized by an interval where the inductive devices (i.e., the windings) in the switched mode power supply reach zero energy (i.e., the current passing through the windings drops to zero), and remain there until the next switching cycle.
But as the time increases during which a primary winding inductor is in a zero energy state, the cross regulation between the multiple outputs of the power supply worsens. Recognize that the only time that all of the outputs can be coupled is during intervals when there is energy stored in the magnetic field of the inductor or transformer. In flyback converters, the only interval that all of the outputs can be coupled is when energy is being passed from the transformer to the output capacitors. In other words, when the main primary winding in the power converter reaches zero energy, the voltages of the other windings begin to drift. Lighter loads and lower switching frequency cause the percentage of time that the outputs are coupled to become lower which will allow the output voltages to drift from a relationship that is defined by the turn ratio of their windings. This can be of particular concern when the voltages of the other windings are used to power specific circuits that require a certain minimum voltage to operate. In such cases, there is a danger that if the respective voltage drifts too low, it might provide insufficient power to a circuit, requiring it to shut down, enter a low power mode, or perhaps perform a reset function, thus reducing the operational efficiency of the circuit.
This can be of particular concern when one of the converted voltage outputs is used to power the primary side controller, which operates the MOSFET switch.
It would therefore be desirable to improve the cross-regulation of the converted power output voltages in a multiple-output power converter during a light load or discontinuous conduction mode of operation.